In recent electronically-controlled engine control systems, a fuel injection quantity for each cylinder, i.e., an air-fuel ratio of air-fuel mixture supplied to an engine (mixture air-fuel ratio), is feedback-controlled based on an output signal of an air-fuel ratio sensor for detecting an air-fuel ratio of exhaust gas (exhaust air-fuel ratio) so that the exhaust air-fuel ratio is controlled to be within a predetermined air-fuel ratio range including the stoichiometric air-fuel ratio. In this range, an exhaust purifying catalyst can purify exhaust gas with a high purification rate. This type of air-fuel ratio control system has a function of diagnosing abnormality.
For instance, a system disclosed in JP 7-224709 A uses, as abnormality diagnosis data, individual air-fuel ratio feedback correction factors (coefficients) used for individually feedback-controlling mixture air-fuel ratio of multiple cylinders, and checks whether the individual feedback correction factors are within a predetermined range. If any one of the individual feedback correction factors is outside the predetermined range, a cylinder of the engine corresponding to such a feedback correction factor is determined to be abnormal. Since this control system requires the individual feedback correction factors for multiple cylinders as the abnormality diagnosis data, it is not possible to perform the above abnormality diagnosis in other control systems, which do not feedback-control the individual air-fuel ratios of multiple cylinders by calculating individual feedback correction factors.